1. Field of the Invention
The present invention relates to a storage device that reads servo information, which is written in an inserted and held storage medium, with a head and controls a position of the head using the read servo information and a servo information writing method.
2. Description of the Related Art
Conventionally, recording density of storage media such as a magnetic disk, a magneto-optical disk, and an optical disk has been remarkably improved. This makes it possible to realize a reduction in size of a storage device and an increase in a recording capacity. Convenience of the storage device is improving.
In such storage media with the improved recording density, to accurately position a head on a target track, servo information such as a track number and a servo pattern (a servo mark) for demodulating a relative position of the head with respect to a track center is written by a servo track writer (STW). The STW writes the servo information from one direction on an outer side or an inner side of a storage medium. A control device such as a magnetic disk device performs, using the servo information, automatic control of a head that reads data from and writes data in the storage medium (see Japanese Patent No. 2645182).
In the writing of the servo information by the STW, deterioration in a quality of servo written in the storage medium due to erase called a side fringe occurs. Problems such as a servo mark mistake and misdetection of a Gray code during reading of the servo information by the head occur. The side fringe is caused by magnetic field leakage (write spread) of a write head that writes the servo information in the storage medium. As shown in FIG. 11, the side fringe is closely related to a yaw angle and a feeding direction of the STW. Specifically, as shown in FIGS. 12A to 12C, when the write head writes the servo information in an outer direction, the influence of the side fringe is large on the outer side. When the write head writes the servo information in an inner direction, the influence of the side fringe is large on the inner side. Therefore, an erase section enlarges and the servo quality is substantially deteriorated. In recent years, amid a further increase in track per inch (TPI), deterioration in an error rate of the servo information and deterioration in a position quality of the head due to the deterioration in the servo quality are becoming innegligible. FIG. 11 is a diagram of a relation between the yaw angle and the feeding direction of the STW. FIGS. 12A to 12C are diagrams for explaining the erase portions in the feeding direction of the STW.
As a method of preventing the deterioration in an error rate, the deterioration in a position quality, and the like due to the deterioration in the servo quality, a bidirectional STW that writes, on the outer side, the servo information while feeding an STW head to the inner side and writes, on the inner side, the servo information while feeding the STW head to the outer side is used. When the bidirectional STW is used, a boundary between the servo information written from the outer side and the servo information written from the inner side is present. This boundary shifts in a time direction and a radial direction because of a change in an environment or the like in writing the servo information. The shift causes non-detection or misdetection of servo. To use a storage medium in which the servo information is written by using the bidirectional STW, it is necessary to accurately read out the servo information written in this boundary (a cylinder as the boundary: a boundary cylinder).
However, when the storage medium in which the servo information is written by the bidirectional STW of the conventional technology described above is used, the boundary of the servo information written from both the directions is separated into two boundaries by a change in an environment or the like in writing the servo information. In other words, a boundary cannot be formed in a Gray code as intended and a position of the head for reading out or writing data cannot be correctly controlled.